A number of different systems and methods have been developed for delivering electrical shocks or pulses to a patient's heart for the treatment of detected abnormal rapid heart rhythms (tachyarrhythmias). These methods deliver shocks having specific waveform shapes or sequences to the heart in order to treat the detected arrhythmia by depolarizing the cardiac tissue cells. Tacker, Jr. discloses the use of sequential shocks delivered through multiple pairs of electrodes in U.S. Pat. No. 4,708,145. In Tacker, Jr., a series of rectangular or truncated exponential shocks are delivered to the heart using at least three epicardial electrodes. A first shock is sent through a first pair of the three electrodes and then a second shock is sent through a second, different pair of the electrodes. These shocks are delivered with a time separation of between 0.1 and 2 milliseconds with the preferred separation being 0.5 milliseconds. The purpose of the sequential shocks is to reduce the overall energy delivery requirements for defibrillation. A similar system using endocardial and/or subcutaneous electrodes is described in U.S. Pat. No. 4,727,877 to Kallok.
The use of two successive defibrillation pulses spaced apart by about 70 to 100 milliseconds is discussed by Province, et al. in "Effects of Defibrillation Shock Energy and Timing on 3-D Computer Model of Heart", Annals of Biomed. Eng., vol. 21, pp. 19-31, 1993. In each case, a predetermined delay time is used between the first pulse and the second pulse.
The use of shocks having multiphasic waveforms is described in Jones et al., U.S. Pat. No. 4,637,397. A triphasic shock waveform is disclosed which has three pulses of alternating positive and negative polarity. U.S. Pat. No. 4,850,357 to Bach, Jr. describes the use of biphasic waveforms to defibrillate the heart. Both Jones et al. and Bach, Jr. deliver the initial portion of the shocks simultaneous with the peak of a cardiac complex.
Defibrillation shocks of the type described above are typically in the range of from about 200 to 800 volts delivered as a capacitive discharge from a source capacitance of about 100 to 150 microfarads for a time of from about 2 to 12 milliseconds. Overall energy delivery to the heart for a defibrillation shocks may typically be from about 5 to 40 joules. A monophasic defibrillation shock may typically be a truncated exponential decay with an initial voltage of about 700 volts and a duration of about 6 to 8 milliseconds. A biphasic defibrillation shock may typically have an initial positive phase of about 700 volts for a duration of 6 milliseconds and a negative phase of about 100 to 200 volts for an equal duration. The leading edge voltage of the second phase of a biphasic shock is typically equal to or one half of the trailing edge voltage of the initial phase which itself depends on the tilt of the pulse. The overall energy delivered is a function of the initial voltage, duration, source capacitance and lead impedance.
A pathological tachycardia is one form of tachyarrhythmia which is characterized by rapid but organized heart rhythms. It typically has a rate from about 110 beats per minute (bpm) to about 190 bpm, depending on the patient, and may be treated with cardioversion pulses. These shocks are similar to defibrillation shocks but generally are delivered at lower voltages, and are delivered simultaneously with the QRS complex. A single shock is delivered simultaneous with the QRS complex to help avoid accelerating a heart experiencing a ventricular tachycardia into ventricular fibrillation. Such a cardioverter is disclosed in U.S. Pat. No. 4,384,585 to Zipes. In Zipes, an implantable intracardiac cardioverter detects intrinsic depolarization of cardiac tissue and provides a single shock to the heart simultaneously with the detected cardiac activity at a time when the bulk of cardiac tissue is already depolarized and in a refractory state. According to Zipes, this reduces the energy needed for cardioversion.
Another technique for treating a pathological tachycardia is to deliver a sequence of from about three to fifteen antitachycardia pacing pulses. The first pulse in the sequence is delivered simultaneous with the R-wave of a QRS complex and the subsequent pulses are delivered with a predetermined spacing between the pulses which may be constant within a given sequence or may vary. Such a system is disclosed in U.S. Pat. No. 4,398,536 to Nappholz et al.
In European Patent Application No. 0 540 266 A1, Kroll et al. disclose a system for delivering one or more pretreatment shocks to a fibrillating heart. They explain that such pulses begin the process of organizing the action of the chaotically contracting myocardial cells so that the defibrillating shock applied after the pretreatment can accomplish its task with less energy than would otherwise be required. The pretreatment may include a train of shocks of appreciably lower energy than the defibrillation shock with various predetermined inter-pulse times. In one disclosed embodiment, a train of short duration, high voltage pretreatment shocks are followed by a high voltage defibrillation shock. The spacing between each of the pretreatment shocks and also the defibrillation shock is set at 200 milliseconds. This spacing is selected because it corresponds to a heart rate of 300 beats per minute and because it allows sufficient time for recharging of the high voltage capacitor after each shock.
In U.S. Pat. No. 5,282,836 to Kreyenhagen et al., an implantable atrial defibrillator is disclosed which provides pre-cardioversion pacing to stabilize the cardiac rate of the heart prior to the application of the cardioverting electrical energy. The rate for the stabilizing pacing is determined by averaging a last preselected number of cardiac cycles. In one embodiment, if an early ventricular activation is detected, the pacing rate is redetermined based on the more recent cardiac cycles. Typically, eight pacing pulses are delivered to the ventricle prior to delivering a cardioverting shock to the atrium.
Some prior art defibrillators deliver defibrillation shocks to the heart without any correlation or synchronization to the timing of the sensed QRS complex from an electrocardiogram (ECG). Other prior art devices synchronize such shocks to the QRS complex.
A technique for delivering a sequence of defibrillating pulses which are in-phase with the sensed ECG signal is described in U.S. application Ser. No. 114,574 filed Aug. 31, 1993 which is the parent of the present application. The ECG is continuously sensed during the application of the defibrillating output and each output is held constant until a threshold crossing of the ECG is detected, whereupon the defibrillating output is changed.
A primary goal in treating a detected tachyarrhythmia with an implantable cardioverter/defibrillator is to ensure delivery of effective therapy while minimizing energy delivery requirements for defibrillation or cardioversion. Lower voltage therapy is less painful and disruptive to the patient. Also, lower voltage electrical pulses allow for use of smaller batteries and capacitors even where the overall energy delivery is not reduced. Smaller batteries and capacitors result in a smaller implantable defibrillator and thus improved patient comfort.